1. Field of the Invention
The field of the invention relates to data processing and in particular to power gated circuitry.
2. Description of the Prior Art
It is known to provide integrated circuits with one or more virtual power rails and one or more virtual ground rails. These virtual rails are selectively connected or disconnected to the main power rails and the main ground rails respectively by header transistors and footer transistors. This technique is useful in reducing power consumption when a block/domain within an integrated circuit is not required to be active and accordingly can be powered down and isolated from the power supply and the ground by the use of these header and/or footer transistors. These header and/or footer devices are selected such that when they are switched off they have a high resistance and thus, a low leakage current. This is generally done by selecting devices with a high threshold voltage.
Conventionally when a circuit is power gated in this way, it may be necessary to divide the circuit into portions that can be individually powered up in a sequential fashion to avoid a large in-rush current on power up. Such a current can cause physical damage to the circuit or may cause sensitive circuitry in the proximity to be perturbed by the drop in voltage that such a current produces.
Although the division of the circuit in this way addresses the in-rush current problem, it brings its own drawbacks. In particular, the provision of what are in effect several isolated virtual power supplies can cause the circuit to be more sensitive to manufacturing defects and these can lead to variations in the performance of the circuit portions power gated by different devices. This is due to differences in the power gating devices leading to differences in the voltage and/or current levels supplied to the circuit portions. In particular, if the supply and demand on each isolated virtual power supply has not been correctly balanced the performance of the power-gated circuit portions may suffer as they may not be supplied with sufficient current to function optimally.
FIG. 1a shows an integrated circuit 5 according to the prior art. The integrated circuit 5 comprises a first voltage rail 10 connected to a positive voltage source Vdd and a second voltage rail 20 connected to Vss. Processing circuitry 30 is arranged between these two voltage rails and is powered by them. Circuitry 30 is divided into separate portions 32 and 34 which are each individually power gated by PMOS power transistors 42 and 44 respectively. Although, in this figure only two portions of the circuitry 30 are shown it will be clear to a skilled person that in general there are many portions. Integrated circuit 5 also comprises control logic 50 for controlling the power gating transistors 42 and 44 which act to supply power to their respective circuit portions when on and to isolate them from the voltage source Vdd and thereby turn them off when switched off. Thus, these power transistors can be used to turn the portions of the circuit on and off individually and thereby save power when these portions of the circuitry 30 are not being used. Power gating PMOS transistors are used as these have a high threshold voltage and therefore low leakage current.
Circuitry 30 is divided into portions which are each separately controlled to provide some control over the current flow that will occur when the circuitry 30 is switched on. If circuitry 30 is relatively large and is all switched on at once then there is a large in-rush current which can cause problems to individual devices, can cause a droop in the voltage rail Vdd and can potentially effect other nearby circuits. Thus, the switching on of these circuits is controlled such that different portions are switched on one after the other therefore limiting the in-rush current.
FIG. 2a shows the integrated circuit 5 of FIG. 1a in layout form. It shows the various layers and the contacts laid down on the silicon used to produce this circuit.
In the very different field of sense amplifiers where a circuit is balanced such that it is sensitive to small changes in say voltage, differences in manufacturing tolerances of the devices forming the sense amplifier can lead to the device not being well balanced. To address this problem a voltage equalisation device may be used between the two balanced devices, this acts to ensure that pairs of wires within a critical circuit are held closer to the same voltage than might be obtained if the equalisation device were not present.